The control of a switched reluctance machine can be subdivided into two notional stages. At lower speeds it is possible, within the time in which a rotor passes through its conduction angle with respect to a stator pole, to `chop` control the phase winding current by turning the controlling switch arrangement on and off during the conduction angle. At higher speeds chop control gives way to `single-pulse`, control in which the current is either rising or falling within the conduction angle and there is no opportunity for chopping to take place.
While torque output is the ultimately controlled parameter, it is particularly important to monitor the phase winding currents in the machine to maintain effective control of the drive. While it is possible to derive a current reading from a direct electrical connection to each phase winding, it is likely to be safer in most situations to use electrically isolated techniques because of the high voltages at which the phase winding operates. To this end, it is known to use devices such as Hall-effect current sensors and flux nulling sensors to achieve a current reading. A suitable flux nulling sensor is the LT200-S manufactured by LEM S.A. of Geneva, Switzerland. It has also been proposed to use a Rogowski coil technique to derive a measure of current, for example in the manner disclosed in British Patent Application No. GB 2259150A.
Whereas it has been possible to replace the electrically connected techniques with the isolated sensors described above, it still requires the use of a single sensor per phase winding in a multiphase machine. An example of current sensors individual to each phase winding in a switched reluctance machine is shown in EP-A-0573198.